The present invention is directed to a process and apparatus for precisely metering quantities of granular or pulverulent solids to positions or locations of utilization of such solids.
The present invention is more particularly directed to such a process and apparatus for precisely metering quantities of granular or pulverulent solid fuel to a burner.
Thus, while in its broadest aspects the present invention is directed to a process and apparatus for precisely metering quantities of any type of granular or pulverulent solid material, such as sand or various powdered or granulated chemicals, the present invention is particularly directed to such a process and apparatus for precisely metering small quantities of granular or pulverulent solid fuels, such as coal, lignite, or wood wastes, to burners of tunnel kilns, heat treating furnaces, blast furnaces, fluid bed combustors, and other known combustion devices where a number of relatively small, independently controlled burners are required.
Although various systems exist for controlling the feed of large quantities of pulverized solid fuel to large burners such as boilers, only a relatively few types of systems are available for metering such fuels to a number of small burners. Such known systems for metering granular or pulverulent solid fuel to a number of small burners may be generally classified as either mechanical or pneumatic devices.
Such known devices of the mechanical type require moving parts in contact with the granular or pulverulent solid material and are thus subject to wear, jamming, breakdown and high maintenance and operating costs. Control of the fuel feed is non-linear and is not readily calibrated, such that repeatability of precise fuel feed control is difficult. The capital cost of such mechanical devices is high.
Among the known types of pneumatic devices are air gravity conveyor systems. However, this type of system does not allow for independent control of individual burners. Thus, by the very nature of operation of this type of device, a change in the feed rate to one burner will noticeably change the feed rate to all down stream burners. Furthermore, the feed of the granular or pulverulent fuel is subject to waving and pulsing as a function of time, is not easily repeatable, and therefore cannot accurately be calibrated.
Further known systems of the pneumatic type are evidenced by U.S. Pat. No. 1,305,726 to Leonard et al., U.S. Pat. No. 4,002,372 to Edwards et al., and U.S. Pat. No. 4,085,976 to Edwards et al. These systems achieve feed of a granular fuel by creating suction in a feed line. In this type of known system, however, a change in the supply of combustion air line will directly effect the amount of fuel supplied. Furthermore, the system according to the two Edwards et al. patents provides for a small air stream to blow across an angle of repose of a pile of granular coal and pick-up particles of coal from the side or angle of repose. Thus, the supply of fuel is dependent upon the pile of fuel having a constant angle of repose. However, since the angle of repose of the pile of fuel particles in fact inevitably changes during the passage of time, the feed of fuel from this type of system pulses or waves as the bed is depleted an then replenished. This pulsing or waving produces a significant effect on the operation of the burner, since the overall fuel flow to the burner is relatively small. Due to this pulsing, it is not possible to achieve precise on-ratio combustion or below ratio turn down of the parameters.
Further known systems of the pneumatic type are evidenced by U.S. Pat. No. 4,092,094 to Lingl and U.S. Pat. No. 4,131,072 to Lingl. These known systems provide for a flow of combustion air passing through an angle of repose of a pile of granular fuel. Thus, these systems suffer from the inherent disadvantages of pulsing and waving due to changes in the angle of repose in the pile of coal. Also, since in these systems it is the combustion air itself which feeds the granular or pulverulent fuel, any change in the supply of combustion air has a obvious direct effect on the supply of fuel.
Those systems supplying fuel from or through an angle of repose of a pile of fuel are particularly subject to variations in flow rate of the fuel supplied, since the angle of repose will inherently be a function of the density, particle size, particle shape, and moisture content of the particular fuel particles, and even a small change in any of these parameters will substantially effect the angle of repose and thus the flow rate of the fuel supplied.